Method for monitoring the production of elongate profiles

ABSTRACT

A method particularly suitable of monitoring the production of elongate profiles which are formed from one or more mixtures of rubber or thermoplastic material in an extrusion head which is fed by one or more extrusion devices is disclosed as in an apparatus which is particularly suitable for carrying out such a method. During production, the strip passes through a cooling path or station in which longitudinal shrinkage thereof occurs. The desired shrinkge is known and the actual shrinkage is monitored by suitable means. These means are operatively connected to a regulating or control device. When leaving the head, the profile passes over a weighing device which determines the weight per unit length thereof. The weighing device is also operatively connected to the regulating or control device. If a deviation from the desired shrinkage in the zone or station is ascertained, the regulating device sets a new, corrected, desired weight per unit length value to be achieved at the weighing device. To achieve this desired weight per unit length, the drives for the extrusion devices are each adjusted by a uniform mount by the regulating or control device. By achieving the desired weight value, the desired amount of shrinkage in the cooling path or station is achieved.

FIELD OF THE INVENTION

The present invention relates to a method for monitoring the productionof elongate profiles and to an apparatus suitable for carrying out sucha method. More particularly, the monitoring method and apparatus of thepresent invention are designed for use when the elongate profiles areformed from one or more mixtures of rubber or thermoplastics materials.

BACKGROUND OF THE INVENTION AND PRIOR ART DISCUSSION

The production of elongate profiles from one or more mixtures of rubberor thermoplastics materials utilising an extrusion head to which themixture or mixtures are supplied by one or more extrusion devices isknown. It is also known to cool the profile by conveying it through acooling zone comprising a plurality of conveyor belts. An apparatus usedfor producing a profile in the form of a tread strip from two differentrubber mixtures is disclosed in German Patent Specification No.2,201,631. In such prior specification, two separate screw presses,which are disposed one above the other, feed different rubber mixturesto an extrusion head where the material mixtures are combined andextruded therefrom in the form of a tread strip. U.S. Pat. No. 2,382,177also shows such an arrangement.

The extruded tread strip is then conducted over a tread strip weighingdevice. Such weighing is achieved by weighing the amount of materiallocated on a rotating conveyor belt. The desired weight of the treadstrip is known and if the actual weight deviates therefrom, a certaindegree of correction can be achieved. Thus, if the actual weight ishigher than the desired weight, the rate of transfer of the tread stripcan be increased. This effectively causes a weight reduction to beachieved because the strip is elongated.

In practice, therefore, such apparatuses for producing tread strips havebeen operated at discharge rate which is slightly in excess of a desiredrate. The extruded tread strip is then brought to its desired weight asa result of the elongation.

The only regulation of the characteristics of the tread strip profileoffered by such an arrangement is that the extruded tread strip can beelongated or compressed by increasing or reducing the rate of transferof the strip. However, the rate at which the tread strip is dischargedfrom the nozzle must be considered as the minimum rate for the treadstrip transfer apparatus and, for the purpose of regulating the profilethickness, it is vital that the rate of transfer does not fall belowthis minimum rate.

U.S. Pat. No. 4,097,566 also discloses a method and apparatus forproducing profiles formed from a plurality of mixtures. The method whichis described therein is highly complex having regard to the regulatingtechniques employed and, additionally, has one fundamental disadvantage.If, for exaple, one of the weighing devices or one of the measuringdevices used for measuring the width of a deflection loop ascertainsthat the produced profile does not have the desired dimensions,adjustment can still only be effected by either extending or compressingthe profile strip.

Each extension or elongation operation on the profile strip, produced,for example, by accelerating the conveyor belts, provides the profilestrip with the correct weight but with totally inaccurate anduncontrolled dimensions. This is evidenced by the uncontrolled andirregular longitudinal shrinkage which occurs when the profile strippasses through the cooling zone.

An additional disadvantage of such a known arrangement resides in thefact that the apparatus used is very long. Accordingly, from the timewhen the deviation from the desired weight of the profile strip isascertained to the time of its correction, a large amount of unusablematerial has passed through the apparatus. This is because a largeamount of profile strip passes between the particular measuring pointand the source of error, that is to say, the point of deviation from thedesired value. Such profile strip portion is, therefore, useless.

Basically, it must be stressed that it is not feasible to extend anextruded profile to compensate for variations in thickness of theprofile because each extension operation produces, during cooling,longitudinal shrinkage which is too inaccurate and too uncontrollable.

OBJECTS OF THE INVENTION

The present invention seeks to provide a method and apparatus wherebythe disadvantages of the above-described methods and apparatuses areovercome. In particular, the present invention seeks to ensure thatdeviations from the desired shrinkage value can be corrected veryrapidly, so that only a minimum length of the profile is wasted.

In order to achieve such an aim, it is clear that actual physical actionupon the profile strip which has been produced is totally unsuitable.Thus, as mentioned hereinbefore, elongation of the profile strip is anunsuitable way for correcting deviations in the thickness because eachextension or pulling operation results in uncontrollable regressiveshrinkage phenomena as the profile cools.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method ofmonitoring the production of elongate profiles formed from one or moremixtures of rubber or thermoplastics material in an extrusion head whichis fed by one or more extrusion devices, the extruded profile beingconveyed through a cooling path, comprising a plurality of conveyorbelts, wherein the longitudinal shrinkage of the profile strip whichoccurs in the cooling path is monitored, the monitored values beingtransmitted to a regulating or control device and being compared with anestimated longitudinal shrinkage value, the control device beingconnected to a weighing device for measuring the weight per unit lengthof the strip and to individual drive means for each of the extrusiondevices whereby, if the monitored shrinkage value deviates from theestimated value the regulating or control device sets a correcteddesired weight per unit length value at the weighing device anduniformly influences each of the drive devices of the extrusion devicesso as to cause the extruded strip to attain the corrected desired weightper unit length so as to ensure that the desired shrinkage in thecooling zone is achieved.

Also according to the present invention, there is provided an apparatusfor monitoring the production of elongate profiles formed from one ormore mixtures of rubber or thermoplastics materials comprising anextrusion head fed by one or more extrusion devices, each extrusiondevice having an adjustable drive, a regulating or control devicecontrolled by a microprocessor, a device for measuring the weight perunit length of the profile and a cooling station which comprises aplurality of conveyor belts disposed one above the other and measuringmeans for measuring the curvature of the loop of the profile strip as itpasses from one conveyor belt to the next, the measuring device formeasuring the carvature of the loops of the profile being operativelyconnected to the regulating and control device, the regulating devicealso being operatively connected to the weighing device, the weighingdevice being disposed at the outlet of the profile strip extruder head,and to the adjustable drives of the extrusion devices.

By scanning the actual shrinkage values obtained in the cooling station,which may be up to 1.5% of the length of the profile, and then comparingsuch values with the desired shrinkage values by means of the regulatingand control device which may, for example, be a microprocessor, theshrinkage values are determined from measuring point to measuring point.

Any deviation from the desired shrinkage values which are detected theregulating or control device during the cooling operation cause theregulating or control device to act upon the weighing device fordetermining weight per unit length of the profile, which weighing deviceis disposed at the outlet from the extrusion head, that is to say, thenozzle from which the profile issues. The weighing device receives, fromthe regulating or control device, a new desired weight value which hasbeen upwardly or downwardly corrected accordingly.

This new desired value is obtained by simultaneously controlling theadjustable drives of each of the extrusion devices, such control canalso be effected by means of the regulating or control device. Thiscontrol is, of course, in strict dependence upon the new desired valuetransmitted to the weighing device by the regulating or control means.

If, for example, instead of the desired shrinkage of 1.5%, a shrinkageof 0.5% is ascertained in the cooling zone, such information istransmitted to the suitably programmed regulating or control devicewhich, in turn, increases the desired weight per unit length at theweighing device. The amount of such increase is empirically determinedand is expressed in the programming of the regulating or control device.

Similarly, if, for example, a shrinkage of 2% is ascertained, that is tosay, 0.5% above the desired shrinkage value, the regulating devicereduces the desired weight per unit length at the metric weighingmachine by a suitable amount. This desired weight is also empiricallydetermined and fixed in the program of the regulating or control device.

In either case, the actual weight per unit length is caused tocorrespond to the new desired weight per unit length by appropriatelyincreasing or reducing the rate of drive of each of the extrusiondevices. Such increases or decreases are by uniform percentages and arecaused by the regulating or control device which is operably connectedto each of the drives.

A reduction in the rate of drive of the extrusion device results in areduction in the discharge therefrom and, accordingly, in a reduction inthe weight per unit length. Similarly, an increase in the rate of drivewill cause an increase in the weight per unit length.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of an apparatus in accordance with the present inventionwill be further described, solely by way of example, with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic view of the embodiment of the apparatus,

FIG. 2 is a schematic detail of part of the apparatus shown in FIG. 1but on an enlarged scale and shows measuring devices in a cooling zone;

FIG. 3 is a schematic detail view of a further part of the apparatusshown in FIG. 1, again on an enlarged scale and shows three extrusiondevices provided with a regulating or control circuit;and

FIG. 4 shows light strips for detecting the overhead loop in the coolingpath.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, there is shown a multiple extrusion head 1 suitable forproducing, for example, tread strips for vehicle tyres. The head 1 isfed through three nozzles 2, 3 and 4 with different rubber mixtures suchmixtures being supplied to the nozzles 2, 3 and 4 by three separateextrusion devices 5, 6 and 7 respectively.

In the region in which the mixtures enter the nozzles 2, 3 and 4,measuring devices 2a, 3a and 4a are provided for measuring the pressureand temperature at these locations.

Each of these measuring devices 2a, 3a and 4a is connected to aregulating or control device 8.

The extrusion devices 5, 6 and 7 are,obviously, driven and their drivesare connected to the regulating or control device 8. By so doing, thedevice 8 can be utilised for modifying the particular rate of rotationof the screw in each of the extrusion devices.

A tread strip 10, which is formed from the three mixtures, is extrudedfrom the multiple extrusion head 1 and passes over a weighing device 9.The weighing device 9 is also connected to the regulating or controldevice 8 and any deviation from predetermined desired weight of thetread strip per unit length is ascertained by the regulating device 8.Thus, for example, too low a weight per unit length indicates that toolittle material is being extruded from at least one of the nozzles 2, 3and 4.

Deviations from predetermined desired values of the pressure andtemperature in the extrusion devices 5, 6 and 7 are likewise detected bythe measuring devices 2a, 3a and 4a and are transmitted to theregulating or control device 8.

If, for example, the weight per unit length determined by the weighingdevice 9 is ascertained as being too low by the regulating or controldevice 8, the regulating device 8 monitors the pressure and temperaturevalues in the individual nozzles 2, 3 and 4 and compares them with thedesired values.

If the regulating device 8 then ascertains that, for example, thepressure and the temperature in the nozzle 2 have dropped, thisindicates that the extrusion device 5 is conveying less material thanthe prescribed amount. Accordingly, the tread strip, which is producedfrom all three mixtures, will have a lighter weight per unit length.

The temperature must also be ascertained because, with an increase intemperature, the material viscosity drops. In other words, the materialbecomes more fluid and the measured pressure of the material will dropalthough the quantity of material being conveyed does not decrease. Inconsequence, the weighing device does not ascertain any change in weightper unit length. However, if the scanned pressure drops without thetemperature rising, this definitely indicates that the quantity ofmaterial conveyed through the screw of the extrusion device hasdecreased.

If, therefore, the scanned value for both the pressure and thetemperature drop by at least a predetermined extent relative to thedesired values, and such changes are ascertained by the regulatingdevice, the rate of rotation of the appropriate screw is adjusted by thedevice 8.

The weight per unit length of the tread strip produced is, of course,lowered by an amount which corresponds to the amount of underfeed of thematerial by the extrusion device 5. Because both the weight per unitlength of the tread strip and the pressure and the temperature, measuredby measuring means 2a, in the nozzle 2 fed by the extrusion device 5 aretoo low, the regulating device 8 accordingly increases the rate ofrotation of the screw in the extrusion device 5 by, for example, 1.5revolutions per minute. The output from the extrusion device 5 is thusslightly increased. Such an increase is continued until thepredetermined desired weight per unit length of the tread strip 10 hasbeen restored.

After leaving the weighing device 9, the tread strip 10 reaches ashrinkage path 11 which, in this embodiment, is formed from twoinclinedly disposed roller paths 11a and 11b.

In practice, approximately 8% of the total shrinkage of the tread stripof a total of about 9.5% is achieved in the shrinkage path 11. Byaltering the angle of inclination of the roller paths 11a and 11b, it ispossible to influence the degree of shrinkage of the tread strip 10 in adesired manner. Thus, if the angle of inclination is large, moreintensive shrinkage occurs and, if it is small, less shrinkage occurs.

Thereafter, the tread strip reaches a second weighing device 12 whichmeasures weight per unit length. The device 12 is connected to a furtherregulating or control device 13. The device 13 is also connected to theroller paths 11a and 11b. Depending on deviations from the predetermineddesired weight per unit length determined by the weighing device 12, theregulating device 13 adjusts the angle of inclination of the rollerpaths 11a and 11b of the shrinkage path 11.

The shrinkage path 11, the metric weighing device 12 and the regulatingdevice 13 thus form an adjustment unit which operates independently ofthe other components.

After traversing the shrinkage path 11, the tread strip 10 reaches acooling path 14 which is formed, for example, from three conveyor belts14a, 14b and 14c disposed one above another.

During the cooling operation, the tread strip 10 is subjected to furthershrinkage, the amount thereof being determined empirically, for example,1.5% per unit length. The conveyor belts 14a, 14b and 14c are driven atfixed speeds which take into accurate account the fact that shrinkage of1.5% should occur without giving rise to the possibility that theconveyor belts will cause elongation of the strip.

In order to determine the actual shrinkage of the tread strip 10 in thecooling path 14, light strips which create a light curtain or pivotalarm members which are connected to potentiometers may be utilised. Suchdevices are installed at one end of each of the conveyor belts 14a and14b in order to control the curvature of the tread strip during itsdeflection from one conveyor belt to the conveyor belt disposedtherebelow. By so doing, the shrinkage, and hence the length of thetread strip is controlled.

In FIG. 2, a portion of the cooling path is shown. In this arrangementpivotal arms 15 are provided, which arms are connected to apotentiometer 16 at their ends mounted on the pivot axis.

When the speed of the conveyor belt 14a has been fixed, the pivotal arm15 will be located in its central position as shown in full lines inFIG. 2. If, however, the tread strip 10 then shrinks by an amountgreater than the desired 1.5% and for which the speed of the conveyorbelts 14a, 14b and 14c has been set, the pivotal arm 15 will be pulledby the strip 10, into its lower position 17. This pivotal movement isdetermined by the potentiometer 16 which transmits such information tothe weighing device 9.

Such information is transmitted from the weighing device 9 to theregulating or control device 8. The indication, in such a case, is thatthe tread strip is, overall, too heavy.

The regulating device 8 is programmed in such a manner that the rates ofrotation of the screws in the extrusion devices 5, 6 and 7 are eachuniformly but minimally adjusted by approximately 0.5 revolutions perminute. Accordingly, the discharge is slightly increased in a uniformmanner whereby the total shrinkage also decreases accordingly. In otherwords, the tread strip undergoes a slight elongation.

Since the shrinkage path 11 is set to provide a constant 8% shrinkage,the shrinkage is apparent in the cooling path 14 in such a manner thatthe tread strip 10 becomes, for example, 0.5% longer than wouldotherwise be the case. As a consequence thereof, the pivotal arm 15 iscaused to move back into its central position and the tread stripshrinks again in accordance with the values which were initiallypredetermined and for which the speed of the conveyor belts 14a, 14b and14c was set.

The shrinkage values occurring in the cooling path 14 may also bedetected and ascertained by means of light strips, such as those shownin FIG. 4. The mode of operation of such a form of light strips will nowbe described.

Light rays 22 are emitted by a light source 20. Opposite the source 20is a receiver 21 which comprises a plurality of discrete photoelectriccells. If a strip of material 10 is disposed between the light sourceand the receiver, the rays 22a do not reach the associated photoelectriccells. If the strip of material 10 moves in the direction of either ofthe arrows 23 or 23a, the particular photoelectric cells which receivelight from the source 20 are altered. This change is detected by anelectronic evaluation system. In the present case, such movement of thematerial 10 denotes the shrinkage.

I claim:
 1. A method of monitoring shrinkage during production ofelongate profile strips comprised of two or more mixtures of rubberwhich are separately extruded through extruding nozzles and thereaftercombined in an extrusion head to form such profile strip, such profilestrip undergoing substantial shrinkage when cooled after passing fromsaid extrusion head, the method comprising the steps of:(a) providingand storing in a central control unit the predetermined desired weightper unit length of the extruded profile strip and the desired shrinkageof the profile strip in a cooling zone; (b) extruding a strip comprisedof said two or more mixtures, and passing said strip over a weighingdevice to determine the weight per unit length of the strip; (c)comparing the actual weight per unit length of said strip with thedesired weight per unit length of said strip stored in said centralcontrol unit; (d) modifying the feed rate of one or more of saidmixtures to correlate said actual weight per unit length with thedesired weight per unit length; (e) passing said strip through a coolingzone in which longitudinal shrinkage occurs, said strip passing over aplurality of vertically spaced conveyor belts; (f) measuring suchshrinkage in said cooling zone as the strip passes from one conveyorbelt to the next lower conveyor belt, and transmitting such measurementto said central control unit; (g) comparing in said central control unitthe actually measured shrinkage of said strip with the predetermineddesired shrinkage stored in said central control unit; and (h) adjustingthe feed rate of one or more of said mixtures based on the measureddiscrepancy between the actual and desired shrinkage in said strip so asto bring such shrinkage within a predetermined desired value.
 2. Themethod of claim 1, wherein said measuring of the shrinkage of said stripin said cooling zone is effected by the detection of the position ofpivotal arms interposed between superimposed conveyor belts and aroundwhich said strip passes from one belt to the next lower belt, theposition of such arms determining the path of movement of said stripthrough said cooling zone.
 3. The method of claim 1 further includingthe step of first passing said extruded strip through a shrinkage pathcomprised of inclined roller paths over which said strip passes beforepassing to said cooling zone, thereafter passing said strip over asecond weighing device which measures the weight per unit length of thestrip, comparing the weight per unit length with a predetermined desiredweight per unit length, and adjusting the inclination of said rollerpaths based on such comparison.